Sodium bicarbonate production

ABSTRACT

Coarse sodium bicarbonate crystals of controlled size are produced by spraying an aqueous mixture of soda ash and sodium bicarbonate into an atmosphere containing CO2 in a carbonating zone; simultaneously evaporating water from the aqueous mixture; collecting the resulting slurry of bicarbonate crystals in mother liquor in a crystallization zone; removing therefrom a slurry stream of coarse sodium bicarbonate crystals; separating coarse crystals of sodium bicarbonate from said slurry and recycling the resultant mother liquor. The separated crystals are suitably washed, dried and sized to obtain the sodium bicarbonate product.

United States Patent 11 1 1111 3,870,784 Saeman Mar. 11, 1975 [54]SODIUM BICARBONATE PRODUCTION 2,375,922 /1945 Jeremiassew 23 295 2,883,7 419 9 S 29 [75] Inventor: Walter Hamden Com 3,531,248 9l190 423L122 73A C N H 3,647,365 3/1972 Saeman 423/422 1 sslgnee 's w 3,751,560 8/1973Newmann 423/189 Filedi 1972 Primary Examiner-Oscar R. Vertiz 2 pp No 3 57 ASSiSlLIHI Examiner-gary P. Stl'ZlUb Attorney, Agent, or F1rm-RobertL. Andersen Related US. Application Data [63] Continuation-impart ofSer. No. 213,562, Dec. 29, [57] ABSTRACT 1971 abandoned Coarse sodiumbicarbonate crystals of controlled size are produced by spraying anaqueous mixture of soda [52] Cl 423/422 23/273 0 ash and sodiumbicarbonate into an atmosphere con- I t Cl Cold 7/10 taining CO in acarbonating zone; simultaneously I? o t u u 6 1 a 6 s 6 l l v u I 1 l 6a A 4 s a l 6 4 [58] Field of Search 423/422, 419, 425, 189, th I f b423/232 23/283 mg e resu 1n g surry o icar onae crys ls 1n mother l1quor1n a crystallization zone; removmg 56 R f C1 d therefrom a slurry streamof coarse sodium bicarbonl e erences ate crystals; separating coarsecrystals of sodium bi- UNITED STATES PATENTS carbonate from said slurryand recycling the resultant 1,619,336 3/1927 Drewscn 423/232 motherliquor. The separated crystals are suitably 1 3 7/1932 v a 423/189washed, dried and sized to obtain the sodium bicar- 2,142,9l7 1/1939Reich 423/232 bonate product. 2.183.324 12/1939 Reich... 423/2322,256,962 9/1941 Reich 423/232 15 Claims, 4 Drawing Flgures SODA 49/ i,Z? f2 l4 22 j \l f \11 a 2? g 14 zffld s E 5 3 A M 5; 44-15 7a 5a 1/ H040 1 H l 4 2 5 l 4 1 4 43 5! 7 FUEL i l/ AIR COARSE PRODUCT 51 F/NEPRODUCT PATENTEB NARI 1 i975 SHEEI 1 95 SODA ASH PAM 5 ||||l|\/ 9 w w mp 5 r f a o 4 z c M, H 5 w i d, I 5 a a v PATENIEBHARI H975 3.870.784

' snzz ragfg' CIOARSE p/eooucr FINE ppooucr v [IGZ PATENTEBHAR] H975 sumu gr 4 500A ASH 3'. z Z A L w I 4; 4 47 FUEL M 4? 5% All? 8 cog/e55PRODUCT f2 FINE PRODUCT 1 SODIUM BICARBONATE PRODUCTION RELATEDAPPLICATIONS This is a continuation-in-part of application Ser. No.213,562 filed Dec. 29, 1971, entitled Sodium Bicarbonate Production nowabandoned.

BACKGROUND OF THE INVENTION 1. Field of Invention This invention relatesto a process for producing sodium bicarbonate from soda ash or causticby carbonation and particularly to a process for so producing sodiumbicarbonate meeting wide ranges of size specifications under minimumscaling and heat load conditions.

2. Description of the Prior Art Sodium bicarbonate has long beenproduced commercially by the ammonia-soda process and by carbonation ofsoda ash. In prior art processes, however, purity and particle sizecontrol are problems still susceptible of considerable improvement. Itis difficult to devise improvements and to put them into commercial useeconomically. Satisfactory methods for producing coarse granular sodiumbicarbonate, in particular, currently in demand for detergentcompositions and for other purposes, have not been available.Furthermore where processes producing a satisfactory product areavailable, scaling of machinery utilized to manufacture the product is aconstant problem. Frequently the problem has been solved by utilizingtwo carbonation towers and scaling one while operating the other. Thisrepresents a very inefficient use of processing equipment and increasesthe cost of producing the resulting prodnot.

The principal objects of the present invention are (1 to provide acarbonation process for producing granular sodium carbonate in whichcrystal size can be controlled and which can be utilized, for example,to economically produce 'a crystal size up to ZO-mesh (2) to providesuch a process which circumvents the scaling problems discussed aboveand the economic consequences of the prior art solutions.

The present invention minimizes costs of starting materials by efficientuse of low cost sources of CO by utilizing any common form of soda ashincluding trona ash or light soda ash from the ammonia-soda process, orby utilizing caustic as a raw material in lieu of or in addition to sodaash. It further minimizes costs of the process by minimizing the heatload on the system and by promoting more efficient use of machineryutilized in the process.

SUMMARY OF THE INVENTION According to the present invention, a processhas been devised for producing sodium bicarbonate economically incontrollable particle size and with controlled purity by the steps of:

l. Maintaining in a crystallization zone an aqueous mixture of sodiumbicarbonate crystals in a solution of sodium carbonate saturated withsodium bicarbonate;

2. Maintaining in a contiguous carbonating zone above saidcrystallization zone an atmosphere having at least 4 percent carbondioxide;

3. Spraying an aqueous mixture of soda ash and sodium bicarbonate intosaid carbonating zone, removing evaporated water from said absorbingzone and collecting the resulting carbonated mixture in saidcrystallization zone;

4. Removing from carbonated mixture an aqueous stream containingsuspended coarse crystals of sodium bicarbonate;

' 5. Separating said coarse crystals from said aqueous stream andrecycling the resultant mother liquor.

In the preferred embodiment of the process, a second aqueous stream isremoved from the upper portion of the aqueous mixture in theclassification zone; said second stream is heated and fresh soda ash isdissolved therein and the resulting fortified solution is sprayed intothe carbonating zone. This modification is advantageous when said secondaqueous stream contains suspended fine crystals of sodium bicarbonate.

In a first advantageous modification of the process the second aqueousstream is heated in a regenerative heat exchanger; soda ash is dissolvedin the heated solution and the enriched solution is cooled byregenerative heat exchange with the entering stream and the fortifiedsolution is returned to the carbonation zone. This modification isadvantageous when said second aqueous stream contains suspended finecrystals of sodium bicarbonate.

In a second modification, fine, dry soda ash is fed directly into thesodium bicarbonate slurry in the crystallization zone where the addedash dissolves rapidly and does not substantially affect the purity ofthe bicarbonate crystals present in the crystallization zone.

In a third modification of the process aqueous caustic is used in lieuof soda ash in the process and the aboveidentified modifications.

In a further modification of the process aqueous caustic is subjected totwo stage carbonation utilizing recycled mother liquor and carbondioxide vapor exhausted from a'bicarbonate crystallizer-absorber.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 .is a flow chart showing thepreferred embodiment of the invention.

FIG. 2 is a flow chart showing the first modification of the preferredembodiment utilizing a regenetative heat exchanger.

FIG. 3 is a flow chart showing the second modification of the preferredembodiment wherein dry soda ash is introduced directly into thecrystallization zone without dissolution or filtration of feed solution.

FIG. 4 is a flow chart showing the further modification utilizing twostage carbonation of caustic.

DETAILED DESCRIPTION The method of the present invention is illustratedin the accompanying flow sheets. In FIG. 1, fortified but unsaturatedaqueous solution of soda ash is introduced via line 35 through sprays 12into an upper region of in the gas filled carbonation zone 13 of theclassifying crystallizershown generally at 14. Sprays 12 are arrangedandpressure is controlled to promote impinging of the droplets on theroof and walls of crystallizer 14 to provide a flow of unsaturated feedsolution over the roof and down the walls of carbonation zone 13 andinto crystallization zone 11 in order to minimize scale formation.

Crystallization zone 11 and classification zone 15 contain a mixture ofsodium bicarbonate crystals in an aqueous solution of sodium carbonatesaturated and in equilibrium with sodium bicarbonate. As used herein,

crystallization zone is generally defined at the top by the liquid levelbelow carbonation zone 13, and at the bottom by the termination ofconical baffle 77. Appended to the lower edge of the baffle iscylindrical screen 78 which blocks passage of scale fragments too largeto pass through orifices in nozzles 24. Conical baffle 77 and screen 78thus physically separate crystallization zone 11 from classificationzone 15.

The upper portion of liquor in classification zone 15 comprises crystalfree mother liquor and/or mother liquor containing suspended relativelyfine crystals of sodium bicarbonate; the lower portion thereof containsrelatively coarser crystals of sodium bicarbonate.

A stream containing relatively larger volumes are drawn fromcrystallization zone 11, through classification zone 15, through line21, cooled as desired in cooler 22, returned via line 23 to crystallizerl4 and dispersed through nozzles 24 in the lower region of carbonationzone 13. Nozzles 24 are positioned to direct the spray upward andinwardly from the perimeter of the tower to maximize mid-air collisionof droplets moving in opposite directions and to minimize direct wallimpingement of the droplets.

Carbon dioxide is provided in carbonation zone 13 in the form of washedstack gas via line 16. Exit gas is vented via line 17. Additionalcooling is suitably provided if desired by directing a portion of ventgas via line 18 through cooler 19 and recycling it via line 20 with theincoming stack gas.

An aqueous stream containing suspended fines of sodium bicarbonate isremoved from classification zone 15 of crystallizer l4, heated intransit as desired in steam heater 26 and returned to dissolver 29.Fresh soda ash is introduced to dissolver 29 via line 30. Line 31 isprovided for the introduction of fine, solid soda ash as desireddirectly into the liquor in crystallizer l4. Fortified feed solutionleaves dissolver 29 via line 32, passes through filter 33 and istransferred via line 35 to sprays 12.

A slurry of coarse crystals is removed from the lower portion ofclassification zone 15 of crystallizer 14 via line to centrifuge 41.Mother liquor is recycled via line 42 preferably to the classificationzone. Purge line is provided at 43. Suitably, at other times, or using aduplicate of centrifuge 41, a slurry of fines is removed fromclassification zone 15 via line 44 to centrifuge 41. Alternatively, thefines are returned via line 25 to dissolver 29. The centrifuged productis washed with water from line 45 and then transferred via line 46 torotary drier 47 heated by burning fuel and air in burner 48 andtransferring the burner gas via line 49 to drier 47. Effluent gascontaining fines and CO pass via line 50 to fines collector 51 fromwhich fine product is removed via line 52. Effluent gas from finescollector 51 is dehumidified by cool water introduced by line 53 intodehumidifier 54 and the dehumidified gas is vented via line 55 orrecycled to drier 47 via line 56. Water from dehumidifier 54 is returnedvia line 57 and heater 26 to dissolver 29. Crystal sodium bicarbonateproduct is removed from drier 47 via line 58.

The method of the present invention is further illustrated in theaccompanying flowsheet of FIG. 2. As in FIG. 1, fortified butunsaturated aqueous solution of soda ash is introduced via line 35through sprays 12 in the gas filled carbonating zone 13 of crystallizer14. Recirculation of the suspension is as in FIG. 1. Carbon dioxide isprovided and exit gas is vented and/or rccirculated with cooling as inFIG. 1.

Aqueous stream 62 is removed from classification zone 15 and introducedinto regenerative heat exchanger 66. The liquor, heated by vaporcondensation, leaves the heat exchanger 66 via line and is transferredto dissolver 29, additional optional heating as desired is provided byheater 26. Fortified liquor leaves dissolver 29 via line 32, passesthrough filter 33 and leaves via line 64 to the first stage of the lowersection of regenerative heat exchanger 66. A portion of the uncooled,fortified liquor from line 64, heated by dissolution of soda ashtherein, is split off for washing the roof of the absorber via line 35.The effluent liquor, cooled by evaporation in heat exchanger 66, isreturned via line 65 to crystal suspension zone 11 of crystallizer l4.Vapor transfer lines of heat exchanger 66 are shown at 67, 68 and 69.Liquid transfer lines of heat exchanger 66 are shown at 71, 72, 73, and74. A similar heat exchanger is described in Journal of Metals, JulyI966, pages 811-818. Product recovery and drying is as in FIG. 1.

A simpler form of the invention is illustrated in FIG. 3. Fine soda ashis fed directly into the saturated crystallizer liquor via line 31. Anaqueous stream 25 containing suspended fines of sodium bicarbonate isremoved from classification section 15 of crystallizer 14, heated inheater 26 and transferred via line 35 to sprays 12. Recirculation of thecrystallizer liquor and recovery of product is as described in FIG. 1.This modification of the invention eliminates the cost of dissolving thefeed and filtering the feed solution. The cost of the steam fordissolving the feed is also eliminated. The product does not meet foodgrade specifications but capital investment and utility costs arereduced to a minimum.

To meet food grade specification with minimum utility costs, ash must bedissolved and filtered before this stream is admitted to thecrystallizer. The liquor introduced into the feed dissolving circuitfrom the crystallizer must be heated to establish sufficient dilution toserve as a dissolving medium for the ash. Steam to supply this heat is amajor cost item in the production of the crystal bicarbonate. The heatof solution of soda ash in the liquor is moderately exothermic.Therefore, regenerative heating and cooling by efficient countercurrentcontacting of the liquor entering the ash dissolver with the liquorleaving the ash dissolver is utilized. Scaling may occur in theregenerative heater due to the occurrence of supersaturation as theenriched mother liquor is cooled. Heat transfer by vapor exchange is notimpaired by scaling. Scale which forms in a vapor exchanger can also bequickly dissolved by intermittent steaming. Absorber wall scaleformation is minimized by distribution of hot, enriched, but slightlydilute mother liquor from the feed dissolving circuit directly onto theupper walls of the absorber.

To meet food grade specifications with minimum occurrence of scaleformation, all of the hot enriched liquor from the feed dissolvingcircuit is directed to the roof of the absorber. The regenerative heatexchanger in the feed dissolving circuit is therefore omitted. This alsoelminates scaling at this point in the process. Larger amounts of steamare required for heating the liquor in the feed dissolving circuit. Anadditional cooling load is also imposed on the crystallizer since theadded steam load must be removed along with the exothermic heat ofreaction between soda ash and CO However, scale formation in the feeddissolving circuit and on the walls of the absorber is minimized.

If desired caustic may be substituted for soda ash in the modificationsshown by FIGS. l-3 without substantial modification of the systems shownin the flow charts. As used herein the term caustic means caustic sodaor aqueous sodium hydroxide. Thus, in FIG. 1 caustic rather than sodaash would be introduced into the crystallizer via line 31. Mixing of thecaustic with sodium bicarbonate containing liquor converts the causticto sodium carbonate according to the following reactions:

(A) NaOI-I NaHCO Na CO H O (B) Na CO CO H O 2NaI-ICO The exothermic heatof reaction is about 505 Btu/LB. NaI-ICO formed as opposed to about 400Btu/LB. NaI-ICO utilizing soda ash, thus creating an additional heatload on the system which must be counteracted by increasing coolingeffected through cooler 19. The expense of increased cooling issubstantially offset by economies gained in recycling mother liquor tosprays 12. As best seen in FIG. 4, the use of caustic eliminated theneed for steam heater 26. Thus an aqueous stream containing suspendedfines of sodium bicarbonate may be transferred directly to dissolver 29where fines are dissolved upon addition of aqueous caustic introducedinto dissolver 29 via line 30a. Further, the lack of impurities incaustic eliminates the need for filter 33 direct transfer of fortifiedfeed solution from dissolver 29 to sprays 12.

A further modification of the system utilizing caustic in lieu of sodaash is illustrated in FIG. 4. Aqueous caustic is introduced via line 75into a first stage carbonation unit 76 in which it is carbonated to forma finely divided monohydrated crystalline Na CO precipitate. The firststage carbonation is carried out at elevated temperature increasing theefficiency of CO absorbtion over that which can be obtained bypreliminary carbonation in the crystallizer-absorber. Carbonation may beeffected in this first-stage unit either by passing an aqueous causticsolution through an atmospher of CO or by passing CO through the aqueoussolution, both such methods being well known in the art.

Following carbonation the aqueous solution of sodium carbonate andunreacted sodium hydroxide, if any, is introduced into theabsorber-crystallizer via line 31a where sodium bicarbonate is formed inaccordance with steps discussed in connection with FIGS. 1, 2, and 3.

The suspension in the first stage carbonator should contain a ratio of25 pounds of monohydrated Na CQ to 60 pounds saturated ash solution. Dueto the lack of water entering the system in the aqueous caustic (about50 percent H 0) and the high evaporation resulting from high-operatingtemperatures dilute mother liquor is recycled via line 43a to maintainthe ratio.

Operating the first stage absorber at a temperature of up to 90C, asmuch as above the temperature at which the second stageabsorber-crystallizer operates, increasing carbon dioxide absorbtionabout 9 fold. Thus it is desirable to utilize gas vented from the secondstage absorber crystallizer and to introduce this into the absorber 76as a source of CO, for the first stage absorber through line 17a. Theabsorber 76 thus acts as a scrubber for gasses which would normally bevented through a special scrubber from the bicarbonateabsorber-crystallizer. If additional carbon dioxide is required, it maybe supplied via line 16a.

By utilizing the two stage carbonation process described above forprocessing caustic about 60 percent of the heat load is dissipatedthrough the first stage carbonation unit 76, thus avoiding the necessityofincreasing cooling capacity of cooler 19. When coupled with theelimination of heater 26 and filter 33, (shown in FIGS. 1 and 2) theutilization of caustic becomes a practical and economical way to use analternate starting material without sacrificing product quality oroperating efficiency in the process of producing course granular sodiumbicarbonate.

In the process of the present invention, the crystal suspension in thecrystallizer is re-circulated and sprayed into an atmosphere containingat least 4 percent CO in an open absorption column immediately above anexposed crystal suspension. Supersaturation is induced by the absorptionof CO in the carbonatecontaining mother liquor. The sprayed liquor thendrops into the contiguous crystallization zone below.

Fine crystals are segregated and removed from the suspension in theconnecting classification zone as required to balance the amount of seedin the suspension withthe crystal production rate.

Published solubility data for solutions of sodium carbonate and sodiumbicarbonate in equilibrium with sodium bicarbonate crystals show that at45C., for example, the liquor composition may range from a maximum ofabout 18 percent by weight of Na CO and 6 percent of NaHCO to a minimumofO percent Na CO and 12 percent NaI-ICO As the temperature increases,the concentrations of NaHCO required to saturate the solution alsoincrease. At intermediate Na CO concentrations within this solubilityrange the absorption of CO results in a decrease in the concentration ofthe Na CO and an increase in the concentration of NaH- CO according tothe equation:

Nazcog CO H20 Addition to the mother liquor of Na CO either in so lutionor in solid form compensates for carbonate depletion by bicarbonateformation and sustains the Na CO concentration of the mother liquor inthe range indicated above. Na cO in solid form dissolves in the motherliquor and induces crystallization of NaHCO by salting it out.

Commercial grades of sodium bicarbonate crystal are recovered fromthesuspension. Supplemental heat is not required to dissolve the ash andcosts are minimized by elimination of the ash dissolving liquor circuit.

To meet U.S.P. purity requirements, the soda ash feed solution isfiltered to avoid accidental entry of insoluble foreign matter into thesuspension. Heat requirements are minimized if the ash is dissolved inheated mother liquor. Further minimization of heat requirement resultsform regenerative heating and cooling in a countercurrent exchanger ofsolution entering and leaving the feed ash dissolver. Crystal growth isinduced by cooling the enriched feed solution in contact with thesuspension.

Prior art methods for the absorption of gases in liquids are by contactin packed towers, by dispersing liquid in the gas as a spray or cascade,and by dispersing gas bubbles in the liquid. In the prior art methods,scaling due to separation of solids from saturated solutions is aserious problem detrimental to productivity and expensive to combat.

In the process of the present invention, scaling is minimized or avoidedby dispersing the warm aqueous mixture as a spray in an open absorbertower devoid of fixed surfaces in close proximity to one another.Contact time of the spray droplets with the CO in the atmosphere of thetower is limited to avoid excessive supersaturation and the resultingundesirable spontaneous nucleation. Droplet residence time in the COatmosphere is suitably from about 1 to seconds, preferably about 2seconds. Desupersaturation time in the crystallization zone is suitablyfrom 1 to 10 minutes, preferably about 3 minutes. The longer growth timeafforded by the process of this invention in relation to the dropletresidence time for CO absorption results in a reduced rate of crystalgrowth, improved crystal structure and strength. Degradation incentrifuging and drying is thereby substantially avoided. In theabsorber, scaling of the walls is minimized by directing the spraysupwardly and inwardly from opposed wall positions. Direct contact of thedrops with the walls is thereby reduced by mid-air collision betweendrops moving in opposite directions. Descaling of the absorber walls isalso effected by flowing warm unsaturated feed solution over allinternal wall surfaces. In addition, control of the residence time ofthe droplets in the CO absorption space prevents excessivesupersaturation and also contributes to the prevention of scaling in theinterior walls. Minor scaling, if it commences locally, is tolerable onthe smooth interior walls of the absorber where it is not in closeproximity to other surfaces. The absorber is thus insensitive to scaleaccumulations. Due to the simplicity and accessibility of all interiorwall surfaces, the walls are quickly washed with a small amount of waterfrom a spray in a short time if necessary. In this respect the method ofthe present invention is a vast improvement over the conventionalbicarbonate towers of the prior art, with inaccessible and closelyspaced interior surfaces which are extremely sensitive to scaleformation and are difficult to descale.

In the classification zone, according to the present invention, crystalsize is controlled by any suitable means. Advantageously, the methodsfully disclosed, for example, in US. Pat. Nos. 2,856,270 and 2,883,273are suitable. Negligibly small crystals are segregated and removed fromthe suspension in the classification zone as required to maintain asuspension seed rate in balance with the crystal production rate.Conventional bicarbonate towers are devoid of this feature and cannotproduce premium grades of coarse granular sodium bicarbonate.

In the process of the present invention for the production of food gradecrystals, essentially clear mother liquor, free of crystals, except fora negligible quantity of excess fines which may be dispersed therein, iswithdrawn from the classification zone to the dissolving circuit Wheredry sodium carbonate is dissolved therein by heating. The heated,enriched solution is filtered and returned to the absorption zone tomaintain the concentration of Na CO and NaHCO at preferred operatingconcentrations. Cooling of the heated feed solution either before orafter return to the crystallization zone serves to providesupersaturation to promote crystal growth in addition to thesupersaturation provided by absorption of CO Heating of the motherliquor de-saturates the liquor and permits it to be utilized to dissolveadditional sodium carbonate. Heating of the mother liquor also destroysexcess crystal nuclei removed from the elutriation zone. A particularlyadvantageous method of heating and cooling the liquor in the feeddissolving circuit is by passing the solution countercurrently to itselfthrough a regenerative heat exchanger. The heat of solution of dry sodaash is exothermic and the solution leaving the feed dissolving tank ishotter than the solution entering. This differential in temperature isutilized in a regenerative heat exchanger to cool the heated, enrichedfiltered solution returning to the crystallizer by countercurrentcontact with the cooler mother liquor flowing from the crystallizer tothe dissolving tank. The regenerative heat exchanger obviates the needof an additional source of external heat for de-saturating the motherliquor for dissolution of fresh soda ash. The fortified feed solutioncontaining dissolved soda ash is filtered and returned to thecrystallizer directly or via the heat exchanger. The heat requirementsfor dissolving soda ash in the heated mother liquor are much less thenthe heat that would be required for first dissolving the fresh soda ashin water and then providing additional heat for evaporation of thesolvent water so added.

Acceleration of the nucleation rate by artifical means, such as a highspeed attrition impeller operating in the suspension, also requires anassociated increase in the rate of fines removal from the classifyingzone to maintain the coarse crystal growth rate in balance with the seedrate. The fines streams so withdrawn from the crystallizer may then becharged to the centrifuge and drying system and then packaged as fineproduct. Drying is slower and dust losses are higher but this is anadvantageous improvement compared to the production of powderedbicarbonate by milling of larger crystals to meet the demand for finegrades of sodium bicarbonate.

The slurry of larger crystals removed from the crystallizer iscentrifuged to separate the mother liquor from the product crystals. Thecentrifuged crystals are suitably washed and transferred to a rotarydryer where residual moisture is volatilized. In the rotary dryer, thecrystals are dried at relatively low temperature in a C0 atmosphere toprevent decomposition of the bicarbonate to carbonate by loss of C0 Thecentrifuge is suitably a variable speed, automatic-batch type whichminimizes the breakage of crystals and increases the production ofcoarse granular grades. The dry bicarbonate from the dryer istransferred to screening and milling operations by screw conveyors,bucket elevators or by an integral air-veyor system whereby the productis entrained in the dryer exhaust air stream and transported to thescreens by air ducts.

Finer grades of bicarbonate are suitably produced from the coarsergrades grown in the crystallizer by milling and classification in aclosed-loop system.

In the process of the present invention, substantially all of the heatis removed by evaporative cooling of the droplets of dispersed liquor inthe CO absorber, including the exothermic heat of solution of the sodaash, the exothermic heat of crystallization of the bicarbonate, theexothermic heat of absorption of CO and any heat from external sourcesrequired to dissolve the dry soda ash in recycled mother liquor. Thisprocess avoids surface coolers such as shell-and-tube coolers in thesuspension recycle circuit supplying the spray nozzles.

Problems of local supersaturation adjacent to the cooling surfacesresulting in scale formation and impairment of cooler performance areavoided. This permits the supply of unsupersaturated solution in highvolume for deterring the formation of scale on the absorber walls. I

A particular advantage of the process of the present invention residesin providing a large, e.g., to hours of production, dynamic reserve ofsodium bicarbonate crystals in the crystallizer suspension. This reserveserves to slow the rate of crystal growth and to assure the productionof coarse crystals of high strength. Substantial fluctuations in thisreserve over periods of several hours do not adversely affect crystalquality. This reserve stabilizes the production rate over extendedperiods of time and effects economies in other sections of the plant.Intermediate surge silos between major sections of the plant aretherefore unnecessary. The slurry transfer rate from the crystallizer tothe centrifuges is suitably adjusted to accommodate normal batch-wiseoperation. A plurality of centrifuges including a spare unit isadvantageous to minimize the magnitude of surges in crystal flow ratethrough the drier and to assure continuity of flow to the mills in theevent of centrifuge breakdown. In contrast, the reserve of crystals inconventional bicarbonate towers covers only about 2.5 hours of towerproduct. This reserve must be maintained in order to maintainproductivity and cannot be varied to stabilize subsequent plantproduction. Surge silos are required to stabilize flow through screensand mills whereas such silos are necessary in the process of the presentinvention.

CO concentration in the tower suitably varies from 4 to 100 percent,preferably about 8 to 40 percent. Novel nozzle design permits operationwith 4 to 5 percent C0 C0 supplied is suitably boiler stack gascontaining 9 to percent CO or, for example, a plant stream containing 95percent CO and from 0.5-1.5 percent H such as is recovered from ammoniasynthesis plants. Normally gas pressure in the carbonator preventsintake of air and no hazard is presented by the selective absorption ofCO However, suitably provision is made for abnormal conditions byinjecting nitrogen into such hydrogen-containing CO supply.

EXAMPLE I In an absorber-crystallizer feet in diameter and 75 feet higha suitable plurality of spray nozzles are arranged in opposing groups atan elevation of about feet above the bottom of the tower and providespray interaction which increases the rate of CO absorption. Residencetime of the spray droplets is 2 to 3 seconds at a nozzle pressure of 30psig.

Suitable spray nozzles of the 30 to solid cone or hollow cone typehaving rated capacties of to 200 gpm (gallons per minute) operating atdischarge pressures of about 15 to 30 psig provide satisfactoryabsorption rates of C0,. The nozzles are directed to provide maximuminteraction to enhance CO, absorption. Enhancement of the CO, absorptionrate from gases low in CO, concentration is effected by design orselection of nozzles which disperse the recirculated suspension intosmaller droplets. Gas dispersion nozzles are particularly effective ingenerating small drops but also require the dissipation of larger amountof pumping and compression energy.

A funnel-shaped conical baffle is sealed internally to the side wallswith the top of the funnel at an elevation of 25 feet above the bottomof the tower and the bottom of the funnel at an elevation of 15 feet.The bottom of the funnel has a diameter of 12 feet. The concentric zonebetween this funnel and the tower wall provides a quiescent elutriationzone also intended to generate crystal free mother liquor. Ports fittedwith throttle plates near the upper edge of this baffle provide for theregulation of the flow of clarified mother liquor upwardly in theperipheral zone of the baffle.

Suspension circulating pump intakes (2) are provided at an elevation of1 foot above the bottom and these discharge in a battery of spraynozzles similar to those previously described at an elevation of 30 feetabove the bottom. Fragments of scale too large to pass the smallestorifice in the spray nozzles are restrained by a large cylindricalscreen affixed to the bottom of the conical elutriation zone baffle andextending to the bottom of the tower. Recirculated suspension must passthrough the screen to reach the pump intakes. Suspension for supplyingthe crystal centrifuges is preferably drawn from a dynamic suspensionzone in the bottom of the tower.

At an intermediate level 22 feet above the bottom, a stream of crystalfree mother liquor is removed from the elutriation zone and heated inthe regenerative heat exchanger to provide a solution unsaturated insoda ash in which fresh soda ash is dissolved. The regenerative heatexchanger is particularly advantageous in reducing auxiliary steamrequirements for dissolving the ash but cooling is controlled tomaintain the temperature of the returning fortified feed liquor abovesaturation temperature. Connections for drawing mother liquor withvariable concentrations of fine crystal and nuclei are also provided atan intermediate level of about 17 feet. The concentration of crystals inthis stream is dependent on the total rate of flow of crystal-freemother liquor through the elutriation zone. Crystals in this stream maybe recovered as fine product or they may also be destroyed by reheatingin the feed dissolving circuit. a

A suitable rotary dryer is 6 feet in diameter and 25 feet long. It has acapacity of 15,000 pounds per hour of sodium bicarbonate. Hot airenriched with recycle CO entering at 450F. flows cocurrently through thedryer. This minimized air velocity, dust entrainment and bicarbonatedecomposition. The maximum discharge temperature of the bicarbonate isC. (about- 158F.). Internal dryer flighting improves heat transfer andcontributes strength to the dryer shell.

EXAMPLE ll Absorber Operation In an absorber-crystallizer tower having adiameter of 20 feet and an overall height of 76 feet, the cylindricalbody has a height of 44 feet with an 18 foot high conical roof and a 14foot deep conical bottom reducing the diameter to 6 feet. The steep roofangle minimizes scale formation on this surface and the concical bottomminimizes crystal sedimentation on the walls. The top 30 feet of thecylindrical body of the tower is the absorption zone and the bottom 28feet of the tower is the crystallizer and classification zone. Theremaining 18 feet constitutes the conical roof.

Mother liquor containing 13 percent Na CO; and 6.5 percent NaI-ICO wassprayed at a temperature of EXAMPLE III Crude light soda ash was chargedto the stirred dissolver at the rate of 8,620 lb./hr. The soda ash had achemical and mesh analysis as follows:

Chemical analysis Mesh analysis (U.S. Standard Screens) Recycling heatedliquor from the heat exchanger at a temperature of 68C. (154.4F.) wasfed to the dissolver providing a solution containing 24,000 lb./hr. ofNa CO 16,000 lb./hr. of NaHCO and 152,000 lb./hr. of water. Anadditional 4,665 lb./hr. of water was also added to the dissolver, Theeffluent solution from the dissolver, passed through the filter and heatexchanger contained 32,620 lb./hr. of Na CO 16,000 lb./hr. of NaHCO and156,665 lb./hr. of water.

The slurry of fine crystals of NaHCO was removed from the crystallizerat atemperature of 57C. (135F.) and heated in the heat exchanger to 66C.(150.8F.). The exothermic heat of solution of the ash raised thesolution temperatue to 68C. (154.4F.). Meanwhile, hot fortified feedliquor from the filter was charged to the heat exchanger at 68C.(154.4F.) and was cooled to 59C. (138.2F.) before returning to thecrystallizer.

The CO atmosphere in the absorber-crystallizer was provided by feedingwashed stack gas containing 7,997 lb./hr. of CO 5,009 lb./hr. of water,1,683 lb./hr. of O and 42,560 lb./hr. of N The vent gas from theabsorber contained 4,597 lb./hr. of CO 6,457 lb./hr. of water, 1,683lb./hr. of O and 42,560 lb./hr. of N Thus 3,400 lb./hr. of CO wasabsorbed and 1,448 lb./hr. of water was evaporated.

A recycle stream of liquor and crystals from the bottom of thecrystallizer was circulated at a rate of 10,000 gallons per minute tothe lower bank of sprays in the absorber zone to induce absorption of3,400 lb./hr. of CO Coarse crystal slurry was transferred from thebottom of the crystallization zone to the centrifuges and amounted to12,600 lb./hr. of Nal-lCO crystals in 37,600 lb./hr. of mother liquor.The wet bicarbonate removed from the centrifuges amounted to 13,000lb./hr. holding 400 lb./hr. of water. This was dried and screened toprovide 12,600 lb./hr. of coarse, crystalline sodium bicarbonate producthaving a bulk density of 55 lb./ft. It showed the following chemical andmesh analysis:

Component Wt. pct. Size Wt. pct.

NaHCO; 99.8 10+ 20 0.0 Na cO 0.1 20+ 50 10.0 Balance 0.1 50 50.0 l0030.0 --150 200 9.0 -200 1.0 100 100.0

What is claimed is: l. A method for producing sodium bicarbonate bycarbonating sodium carbonate in an absorbercrystallizer to precipitatesodium bicarbonate crystals which comprises: 7

1. maintaining in a crystallization zone a mixture comprising sodiumbicarbonate crystals in an aqueous solution of sodium carbonatesaturated and in equilibrium with sodium bicarbonate;

2. maintaining in a contiguous carbonating zone above saidcrystallization zone an atmosphere having at least 4% carbon dioxide;

3. spraying said mixture upwardly and inwardly into said atmosphere fromthe perimeter of said carbonating. zone, through opposed nozzles adaptedto maximize mid-air impingement of sprays eminating therefrom, removingevaporated water therefrom and collecting the resulting carbonatedmixture in said crystallization zone;

4. removing from said carbonated mixture an aqueous stream containingsuspended crystals of sodium bicarbonate;

5. separating said crystals as product from said aqueous stream andrecycling the resultant mother liquor to said absorber-crystallizer.

2. The method of claim 1 wherein a second stream containing relativelyfine crystals of sodium bicarbonate is removed from said carbonatedmixture, said fine crystals are dissolved in said second stream and saidsecond stream introduced onto the roof and walls of said carbonatingzone.

3. The method of claim 2 in which soda ash or caustic is introduceddirectly into said crystallization zone in amounts sufficient tosubstantially compensate for sodium carbonate converted to sodiumbicarbonate and removed as product and maintain said equilibrium.

4. The process of claim 2 in which said second stream is fortified withsoda ash or caustic.

5. The process of claim 4 in which supplemental soda ash or caustic isintroduced directly into said crystallization zone in amounts which,when added to amounts added to said second stream, are sufficient tosubstantially compensate for sodium carbonate converted to sodiumbicarbonate and removed as product and to maintain said equilibrium.

6. A method for producing sodium bicarbonate by carbonating sodiumcarbonate in an absorbercrystallizer to precipitate sodium bicarbonatecrystals which comprises:

1. maintaining in a crystallization zone a mixture comprising sodiumbicarbonate crystals in an aqueous solution of sodium carbonatesaturated and in equilibrium with sodium bicarbonate;

2. maintaining in a contiguous carbonating zone above saidcrystallization zone an atmosphere having at least 4% carbon dioxide;

3. collecting said mixture in a classification zone and classifying saidmixture into an upper portion having relatively fine crystals of sodiumbicarbonate suspended therein and a lower portion having relativelycoarser crystals of sodium bicarbonate suspended therein;

4. removing a portion of said mixture from said classification zone andspraying the same upwardly and inwardly into said atmosphere from theperimeter of said carbonating zone through nozzles adapted to maximizemid-air impingement of sprays eminating therefrom, removing evaporatedwater therefrom and collecting the resulting carbonated mixture in saidcrystallization zone;

5. removing from said classification zone in an aqueous streamcontaining suspended crystals of sodium bicarbonate;

6. separating said crystals from said aqueous stream as product andrecycling the resultant mother liquor to said absorber-crystallizer.

7. The process of claim 6 in which a second stream containing saidrelatively fine crystals of sodium bicarbonate is removed from saidupper portion, said relatively fine crystals are dissolved in saidsecond stream and said second stream is sprayed into an upper region ofsaid carbonating zone to impinge on the roof and walls thereof toprovide a flow of unsaturated feed solution thereover.

8. The process of claim 7 in which sodium carbonate or aqueous sodiumhydroxide is introduced directly into said crystallization zone inamounts sufficient to substantially compensate for sodium carbonateconverted to sodium bicarbonate and removed as product and to maintainsaid mixture in equilibrium.

9. The process of claim 7 in which said second stream is fortified withsodium carbonate or aqueous sodium hydroxide to provide a flow offortified unsaturated feed solution over the roof and walls of thecarbonation zone.

10. The process of claim 9 in which supplemental sodium carbonate oraqueous sodium hydroxide are introduced directly into saidcrystallization zone in amounts which, added to amounts added to saidsecond stream are sufficient to substantially compensate for sodiumcarbonate converted to sodium bicarbonate and removed as product and tomaintain said mixture in equilibrium.

11. The process of claim 6 wherein the mixture removed from saidclassification zone and sprayed into said carbonation zone is removedfrom a lower portion of said classification zone and contains relativelycoarse crystals of sodium bicarbonate.

12. The process of claim 11 in which a second stream containing saidrelatively fine crystals of sodium bicarbonate is removed from saidupper portion, said relatively fine crystals are dissolved in saidsecond stream and said second stream is sprayed into said carbonatingzone to contact the walls and roof thereof to form a flow of unsaturatedfeed solution thereover.

13. The process of claim 12 in which sodium carbonate or aqueous sodiumhydroxide is introduced directly into said crystallization zone inamounts sufficient to substantially compensate for sodium carbonateconverted to sodium bicarbonate and removed as product and to maintainsaid mixture in equilibrium.

14. The process of claim 12 in which said second stream is fortifiedwith sodium carbonate or aqueous sodium hydroxide to provide a flow offortified unsaturated feed solution over the roof and walls of thecarbonation zone.

15. The process of claim 14 in which supplemental sodium carbonate oraqueous sodium hydroxide is introduced directly into said carbonationzone in amounts which, added to amounts added to said second stream, aresufficient to substantially compensate for sodium carbonate converted tosodium bicarbonate and removed as product and to maintain said mixturein equilibrium.

1. A METHOD FOR PRODUCING SODIUM BICARBONATE BY CARBONATING SODIUM CARBONATE IN AN ABSORBER-CRYSTALLIZER TO PRECIPIATATE SODIUM BICARBONATE CRYSTALS WHICH COMPRISES:
 1. MAINTAINING IN A CRYSTALLIZATION ZONE A MIXTURE COMPRISING SODIUM BICARBONATE CRYSTALS IN AN AQUEOUS SOLUTION OF SODIUM CARBONATE SATURATED AND IN EQUILIBRIUM WITH SODIUM BICARBONATE;
 1. A method for producing sodium bicarbonate by carbonating sodium carbonate in an absorber-crystallizer to precipitate sodium bicarbonate crystals which comprises:
 1. maintaining in a crystallization zone a mixture comprising sodium bicarbonate crystals in an aqueous solution of sodium carbonate saturated and in equilibrium with sodium bicarbonate;
 1. maintaining in a crystallization zone a mixture comprising sodium bicarbonate crystals in an aqueous solution of sodium carbonate saturated and in equilibrium with sodium bicarbonate;
 2. maintaining in a contiguous carbonating zone above said crystallization zone an atmosphere having at least 4% carbon dioxide;
 2. The method of claim 1 wherein a second stream containing relatively fine crystals of sodium bicarbonate is removed from said carbonated mixture, said fine crystals are dissolved in said second stream and said second stream introduced onto the roof and walls of said carbonating zone.
 2. maintaining in a contiguous carbonating zone above said crystallization zone an atmosphere having at least 4% carbon dioxide;
 2. MAINTAINING IN A CONTIGUOUS CARBONATING ZONE ABOVE SAID CRYSTALLIZATION ZONE AN ATMOSPHERE HAVING AT LEAST 4% CARBON DIOXIDE;
 3. SPRAYING SAID MIXTURE UPWARDLY AND INWARDLY INTO SAID ATMOSPHERE FROM THE PERIMETER OF SAID CARBONATING ZONE, THROUGH OPPOSED NOZZLE ADAPTED TO MAXIMIZE MID-AIR IMPINGEMENT OF SPRAYS EMINATING THEREFROM, REMOVING EVAPORATED WATER THEREFROM AND COLLECTING THE RESULTING CARBONATED MIXTURE IN SAID CRYSTALLIZATION ZONE;
 3. spraying said mixture upwardly and inwardly into said atmosphere from the perimeter of said carbonating zone, through opposed nozzles adapted to maximize mid-air impingement of sprays eminating therefrom, removing evaporated water therefrom and collecting the resulting carbonated mixture in said crystallization zone;
 3. The method of claim 2 in which soda ash or caustic is introduced directly into said crystallization zone in amounts sufficient to substantially compensate for sodium carbonate converted to sodium bicarbonate and removed as product and maintain said equilibrium.
 3. collecting said mixture in a classification zone and classifying said mixture into an upper portion having relatively fine crystals of sodium bicarbonate suspended therein and a lower portion having relatively coarser crystals of sodium bicarbonate suspended therein;
 4. The process of claim 2 in which said second stream is fortified with soda ash or caustic.
 4. removing from said carbonated mixture an aqueous stream containing suspended crystals of sodium bicarbonate;
 4. REMOVING FROM SAID CARBONATED MIXTURE AN AQUEOUS STREAM CONTAINING SUSPENDED CRYSTALS OF SODIUM BICARBONATE;
 4. removing a portion of said mixture from said classification zone and spraying the same upwardly and inwardly into said atmosphere from the perimeter of said carbonating zone thRough nozzles adapted to maximize mid-air impingement of sprays eminating therefrom, removing evaporated water therefrom and collecting the resulting carbonated mixture in said crystallization zone;
 5. removing from said classification zone in an aqueous stream containing suspended crystals of sodium bicarbonate;
 5. SEPARATING SAID CRYSTALS AS PRODUCT FROM SAID AQUEOUS STREAM AND RECYCLING THE RESULTANT MOTHER LIQUOR TO SAID ABSORBER-CRYSTALLIZER.
 5. separating said crystals as product from said aqueous stream and recycling the resultant mother liquor to said absorber-crystallizer.
 5. The process of claim 4 in which supplemental soda ash or caustic is introduced directly into said crystallization zone in amounts which, when added to amounts added to said second stream, are sufficient to substantially compensate for sodium carbonate converted to sodium bicarbonate and removed as product and to maintain said equilibrium.
 6. A method for producing sodium bicarbonate by carbonating sodium carbonate in an absorber-crystallizer to precipitate sodium bicarbonate crystals which comprises:
 6. separating said crystals from said aqueous stream as product and recycling the resultant mother liquor to said absorber-crystallizer.
 7. The process of claim 6 in which a second stream containing said relatively fine crystals of sodium bicarbonate is removed from said upper portion, said relatively fine crystals are dissolved in said second stream and said second stream is sprayed into an upper region of said carbonating zone to impinge on the roof and walls thereof to provide a flow of unsaturated feed solution thereover.
 8. The process of claim 7 in which sodium carbonate or aqueous sodium hydroxide is introduced directly into said crystallization zone in amounts sufficient to substantially compensate for sodium carbonate converted to sodium bicarbonate and removed as product and to maintain said mixture in equilibrium.
 9. The process of claim 7 in which said second stream is fortified with sodium carbonate or aqueous sodium hydroxide to provide a flow of fortified unsaturated feed solution over the roof and walls of the carbonation zone.
 10. The process of claim 9 in which supplemental sodium carbonate or aqueous sodium hydroxide are introduced directly into said crystallization zone in amounts which, added to amounts added to said second stream are sufficient to substantially compensate for sodium carbonate converted to sodium bicarbonate and removed as product and to maintain said mixture in equilibrium.
 11. The process of claim 6 wherein the mixture removed from said classification zone and sprayed into said carbonation zone is removed from a lower portion of said classification zone and contains relatively coarse crystals of sodium bicarbonate.
 12. The process of claim 11 in which a second stream containing said relatively fine crystals of sodium bicarbonate is removed from said upper portion, said relatively fine crystals are dissolved in said second stream and said second stream is sprayed into said carbonating zone to contact the walls and roof thereof to form a flow of unsaturated feed solution thereover.
 13. The process of claim 12 in which sodium carbonate or aqueous sodium hydroxide is introduced directly into said crystallization zone in amounts sufficient to substantially compensate for sodium carbonate converted to sodium bicarbonate and removed as product and to maintain said mixture in equilibrium.
 14. The process of claim 12 in which said second stream is fortified with sodium carbonate or aqueous sodium hydroxide to provide a flow of fortified unsaturated feed solution over the roof and walls of the carbonation zone. 